1. Field of the Invention
This invention relates to the field of wireless broadcast of information to a multiplicity of receivers, and in particular to a method and apparatus for increasing the effective information transmission bandwidth or equivalently channel capacity by means of source signal splitting, spatially distributed cochannel transmission, directionally selective reception, and source signal recombination.
1. Prior Art
In wireless broadcast systems, information generated by a source is transmitted by wireless means to a plurality of receivers within a particular service area. The transmission of such information over a fixed time interval requires a finite amount of (frequency) bandwidth, and in current state-of-the-art, transmission of information from different sources must occur in different frequency bands (channels). Since there are quite a few services (e.g., television (TV), FM radio, private and public mobile communications, etc.) competing for a finite amount of available spectrum, the amount of spectrum which can be allocated to each channel is severely limited. Innovative means for using the available spectrum more efficiently are of great value. In current state-of-the-art systems such as broadcast television, a suitably modulated signal is transmitted from a single tower centrally located in the service area and propagate to receiving stations in the service area surrounding the transmitter. The information transmission rate achievable by such broadcast transmission is constrained by the allocated bandwidth (C. P. Sandbank, "Digital Television", John Wiley, 1990 and W. F. Schreiber, "Fundamentals of Electronic Imaging: Some Aspects of Image Processing", Springer Verlag, 1991).
Due to attenuation suffered by signals in wireless propagation, the same frequency channel can be reused in a different geographical service area. Allowable interference levels determine the maximum transmit power at each location, as a well as the minimum separation between service areas using the same channels. Currently, within a service area, the rate at which information can be transferred to the receivers is limited by the fact that use of any channel is on a one-at-a-time basis. Simultaneous transmission of independent signals requires the use of separate channels. The current invention is a new method for increasing the capacity of a broadcast channel several fold by employing directional channels. Directional channels are realized by spatially distributing signals to be transmitted, and employing directionally sensitive receivers, a concept unique to this invention.
In prior art, increasing the capacity of point-to-point communication links has been proposed by S. Anderson, et al. "An Adaptive Array for Mobile Communications Systems", IEEE Trans. Veh. Technology, Vol.40. February 1991, pp. 230-236, and S. C. Swales, et al. "The Realization of a Multi-beam Antenna for Cellular Mobile Land Radio System", Mobile Radio and Personal Communications Conf., U.K., December, 1989, pp. 341-348. These papers addressed means for increasing the number of simultaneous users of a wireless communication system by allowing more than one user to use the same frequency, and exploiting the differences in directions-of-arrival at the receive antenna array to separate the different signals. However, they do not address the problem of increasing capacity in a broadcast channel.
Also in prior art, increasing capacity of point-to-point communication links using directional channels was claimed in R. Roy, et al., "Methods for Estimating Signal Source Locations and Signal Parameters Using an Array of Signal Sensor Pairs," U.S. Pat. No. 4,750,147, March 1985, U.S. Cl. 364-800, and R. Roy, et al., "Methods and Arrangements for Signal Reception and Parameter Estimation," U.S. Pat. No. 4,968,732, July 1987, U.S. Cl. 364-460, and in a recent patent filing, R. Roy, et al., "Spatial Division Multiple Access Wireless Communication Systems", U.S. patent application Ser. No. 806,695, filed Dec. 12, 1991. Therein, no reference is made to the broadcast problem, and increasing broadcast channel capacity is not claimed in these patents. Moreover, in the first two referenced patents, arrays used therein are required to possess a special structure, i.e., sensors occur in pairs of identical elements.
Spatial processing has also been used in the context of spatial diversity techniques wherein multiple antennas that are employed for reception of broadcast signals are scanned for the strongest signal and its output chosen for further processing, or some method for combining the such outputs is applied. Though this leads to a improvement in the quality of the received signal, there is no increase in system capacity.
In prior art, increasing capacity of TV broadcast systems and in particular for HDTV where a severe bandwidth constraint exists, several patents have proposed use of plural/multiple channels to increase information transmission: M. A. Isnardi, et al., "Extended Definition Widescreen Television System Using Plural Signal Transmission" U.S. Pat. No. 4,888,641, October 1988, US Cl. 358-141; T. Okano, "Multichannel Video Signal Transmission/Reproduction System" Japan Patent 5,041,909, December 1987, US Cl. 358-146; E. L. J. Fonsalves, "System of Transmitting High-Definition Television Pictures via a Relatively Narrow Passband Channel, and Also a Transmitter and Receiver Suitable for the System", U.S. Pat. No. 4,935,813, March 1988, U.S. CL. 358-138, W. F. Schreiber. "Definition Television Systems" U.S. Pat. No. 5,021,882, April. 1989, US Cl. 358-141. In W. F. Schreiber, "Reliable Television Transmission Through Analog Channels", U.S. Pat. No. 5,040,211 October 1988, US Cl. 380-14, a system for spatially processing the acquired TV image, dividing it into spatiotemporal frequency components, followed by coding of the information prior to transmission is disclosed. However, none of this prior art mentions the use of spatial processing or directional channels to increase capacity. The word channel in all the above cases refers to frequency channels, and multiple channels require additional spectrum. Reuse of the same frequency channel by transmitters at different spatial locations is unique to this invention.